Repetitively-pulsed lasers having a pulse duration in the range of tens of milliseconds are preferred in a number of laser surgical treatments, both therapeutic and cosmetic. By way of example, such lasers are useful in non-ablative treatments of biological tissue. Such treatments include removal of heavily pigmented skin lesions, such as so-called port-wine stains. A preferred wavelength is a wavelength in the yellow-green region of the spectrum, for example, in a wavelength range between about 500 and 600 nanometers (nm). Long pulse operation allows the delivery of relatively high energy at relatively low peak intensity. This avoids complications due to epidermal damage and microvaporization of blood in vessels. These kinds of treatments are preferably provided by a laser beam extending over a relatively broad area, for example, from about two to ten or more millimeters in diameter. Generally, in such broad area treatments, the larger the area-per-pulse that can be effectively treated, the shorter will be the treatment time and cost, and the deeper will be the penetration of energy and therapeutic fluences.
The area that can be treated in a single pulse is determined, among other considerations, by the energy density (energy-per-unit-area) in the treatment beam required for effective treatment. Accordingly, the higher the absolute energy-per-pulse available in a laser, the broader the area over which it can be distributed at a particular energy density.
Pulse-repetition rate for treatment is preferably greater than 1 Hz. At a slower rate, a surgeon may be required to wait an inordinately long time between an irradiation of one area and an adjacent area. At faster rates, it becomes possible to treat larger areas by moving the treatment beam quasi-continuously.
Considering the above-discussed factors, in designing a laser for pulsed broad-area surgical treatment, an important consideration is simply devising an arrangement for driving the laser, stably, as hard as possible, and as efficiently as possible to provide the fastest areal treatment rate per unit cost (capital and operating) of the laser.
A preferred laser type for providing radiation in the green region of the visible spectrum is an intracavity frequency-doubled solid-state laser wherein a gain-medium such as Nd:YAG provides a fundamental wavelength (1064 nm for Nd:YAG) in the near-infrared (NIR) spectral region. The NIR radiation is frequency-doubled in an optically-nonlinear crystal such as KTP (potassium titanyl phosphate) to provide output-radiation at half the fundamental wavelength (532 nm). Yellow (589 nm) radiation may be generated by intracavity sum-frequency generation by mixing 1064 nm and 1319 nm Nd:YAG radiation.
Operating a pulsed solid-state laser at a pulse duration of tens of milliseconds at relatively low peak power is inherently inefficient, frequency multiplication notwithstanding. At low peak powers second harmonic conversion requires an extremely low loss (&lt;0.5% per round trip) cavity for the fundamental wavelength. The inefficiency problem is further exacerbated as frequency-multiplication or other frequency conversion in optically-nonlinear media, in itself, is an inherently inefficient process.
There is a need to improve efficiency of long pulse intracavity frequency-converted lasers for medical applications. Means of improving such efficiency, of course, should preferably be consistent with above-described medical considerations for optimum pulse-length and energy.